1. Field of the Invention
This invention relates to a photosensitive lithographic printing plate material.
More particularly, it relates to a photosensitive lithographic printing plate material comprising a photosensitive layer having a slip sheet laid on the surface thereof, said slip sheet providing the photosensitive lithographic printing plate with excellent cutting property, being readily removed from the plate after cutting and prior to use of the plate, preventing electrification of the plate which may be caused by removal of a slip sheet, and having good fast-contacting property in the manufacturing process.
2. Description of the Prior Art
Slip sheets for photosensitive printing plate were used originally for the purpose of protecting plate faces. However, since lithographic PS plates which comprises aluminum base plates have been widely used, such slip sheets are required to possess higher performance quality from the viewpoint of manufacture and use.
Thus, from the viewpoint of manufacturing photosensitive printing plate materials, slip sheets should satisfy the following performance requirements.
(1) Photosensitive printing plate materials are generally packaged in units of several tens of plate materials which are piled up. The package process includes the step of cutting such several tens of printing plate materials to a predetermined size with a trimming cutter. In this step, conventional slip sheets damage the edge of the cutter upon several times of cuttings. Once the edge is damaged, the cut area no longer becomes a mirror surface, but it becomes a jagged surface having so great ruggedness that makes further production of the printing plate practically inoperable. PA1 (2) With the spread of lithographic PS plate, automatic mass production systems have been employed to manufacture such plates. In these systems, in general, a slip sheet is laminated to the surface of each individual photosensitive printing plate material and the resulting laminate is conveyed, for example, on a belt conveyor and piled up. PA1 (3) While fast contact of a slip sheet with a plate material is required from the viewpoint of manufacture, a slip sheet having an excessively high fast-contacting property suffers from a disadvantage in that it cannot be readily peeled off prior to use of the photosensitive printing plate material. In such cases, if a strong force is exercised to peel off the slip sheet, the photosensitive layer of the plate material is damaged, which adversely affects image formation. Therefore, it is necessary to readily separate or release slip sheets from printing plate materials prior to use. PA1 (4) Prior to use of photosensitive printing plates, the interposed slip sheets are removed. With prior art slip sheets, removal of slip sheets is accompanied by frictional electrification, which often causes an electric shock, resulting in a decrease in working efficiency. PA1 Therefore, such static electrification of slip sheets must be prevented. PA1 There is a need for a slip sheet satisfying all the above-mentioned requirements. However, such a slip sheet is not known. PA1 Thus, with conventional slip sheets comprised of natural pulp, as previously mentioned, the edge of a cutter is damaged upon several times of cuttings. Synthetic resin-precoated or laminated slip sheets have poor release properties and cause significant generation of static electricity, although use of such slip sheets prolongs the lifetime of a cutting edge. Another slip sheet which has heretofore been developed comprises paper which contains a wax (Japanese Patent Laid-open (Kokai) No. 88062/1980) or a fatty acid or its salt (Japanese Patent Laid-open (Kokai) No. 118041/1980). However, improvement in cutting property of such slip sheet is still unsatisfactory and its antistatic and conveying properties are also insufficient.
Thus, there still remains an intense need for improvement in cutting properties of slip sheets in order to prolong the lifetime of cutting edge. PA2 On this occasion, the slip sheet must have an improved fast-contacting property for the purpose of preventing the slip sheet from shifting out of position or removing from the surface of the plate material. A particular means employed for this purpose is electrification of the laminate, for example, by means of corona discharge. In this treatment, it is necessary to attain adequate fast-contacting property. PA2 Then, from the viewpoint of using photosensitive printing plate materials, slip sheets should meet the following performance requirements.